Sun-sized star ripped apart by massive black hole

Discussion in 'Astronomy, Exobiology, & Cosmology' started by wegs, Oct 28, 2019.

  1. wegs Matter & Pixie Dust Valued Senior Member

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  3. Seattle Valued Senior Member

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    Because God wants it that way.

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    They have the same amount of power (gravity) that the mass that formed the black hole in the first place had. If a small star comes near a large star the same thing would happen. It's the same tidal forces that Jupiter exerts on its closest moon and causes it to always be in a molten state at the crust. The forces are just greater with the black hole.
     
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  5. wegs Matter & Pixie Dust Valued Senior Member

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    Okay, thank you. This makes sense to me. Is it safe to assume that a star is always ''struggling'' against the force of gravity? (to not be destroyed?)
     
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  7. Seattle Valued Senior Member

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    If a star (or anything else) is not near a much larger body then there is little "struggle" going on. Our sun isn't struggling against anything. Our Moon would struggle if it got too close.
     
  8. wegs Matter & Pixie Dust Valued Senior Member

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    I see what you're saying. I'm wondering why the earth doesn't ''struggle'' against the sun's gravity? Why the earth doesn't pull or fall into the sun?

    I didn't know posting this article would stir so many questions inside of me.
     
  9. Seattle Valued Senior Member

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    It's not close enough. Mercury isn't close enough. If Earth wasn't influenced by the Sun at all, it would travel in a straight line right out of our solar system. Instead it revolves around the Sun.
     
  10. wegs Matter & Pixie Dust Valued Senior Member

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    So, the gravity of the sun keeps planets in their orbits. And the moon orbits the earth because of the earth's gravity.

    I like how you say ''influenced.''
     
  11. Seattle Valued Senior Member

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    Angular momentum keeps the planets moving away from the Sun and the Sun's gravity is the counterbalanced force that keeps them coming back.
     
  12. Janus58 Valued Senior Member

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    Gravity's strength varies by the inverse of the distance from the center of the mass concerned. Decrease the distance by 1/2 and gravity increases by a factor of 4. Tidal forces are the result of this. So for example, the side of the Earth nearest the Sun feels a slightly stronger pull of gravity than the far side does. This results in a net force difference across the Earth that stretches it along the Earth-Sun line. The width of the Earth is small compared to the Earth-Sun distance so this force is not strong.
    If we were to move the Earth to an orbit 1/2 as far from the Sun, the Sun's gravity pull on it would increase by 4. But not only that, but the Earth's diameter would make up a larger fraction of the distance. The end result would be that the tidal forces across the Earth would increase by a factor of 8.
    The closer you move in, the stronger the tidal forces trying to stretch the Earth become. With the Sun, the Earth would hit its surface before the tidal forces ever got strong enough to overcome the Earth own gravity holding it together.
    But what if we were to compress the Sun while maintaining its mass? You would be able to move the Earth in closer before hitting its surface and it could experience even stronger tidal forces. Shrink the Sun enough, and you could get the Earth in close enough that tidal force become stronger than its gravity and it will be pulled apart.
    Black holes are large masses squeezed into comparatively small packages. Our own Sun, squeezed into a black hole would be only about 6 km in diameter(across the event horizon). If we were to move the Earth to a distance equal to the Earth-Moon distance from this black hole, the black hole's gravity on the near side would be ~ 95g and on the far side ~83g, for a net difference of 12g pulling it apart, 12 times the surface gravity of the Earth.*

    So the ability for black holes to tear stars apart is due to its small size for its mass, which allows object to get close enough that tidal forces become very extreme.

    * It's actually worse than this. As the Earth gets closer to the BH, the tidal forces stretch it into more and more of an "Egg" shape. This means that the distance between near and far side becomes larger, which increase the gravity differential, making the forces pulling it apart stronger, leading to greater stretching...
     
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  13. James R Just this guy, you know? Staff Member

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    In a sense. The hydrogen that makes up most of a star would collapse under its own gravity in the absence of any other forces. But there are other forces in play. The most relevant one is that the Sun is very hot, so the hydrogen acts like gas under pressure. The pressure forces push outwards, while gravity pulls inwards. At the moment, and for the next few billion years, those two forces are balanced, so the Sun is neither shrinking nor expanding. When the hydrogen starts to run out, though, the Sun will go through a series of interesting changes (e.g. it will initially expand to become a Red Giant star).

    One way to look at it is this: the earth is always falling towards the Sun. It's just that it is moving so fast "sideways" that instead of moving towards the sun it goes around it. The Sun's gravity is constantly pulling on the Earth. If it wasn't, the Earth would not be orbiting in a circle, but would move in a straight line through space.

    Closer to home, satellites (including artificial ones and the Moon) are all gravitationally pulled towards the Earth, but again they are orbiting fast enough that they don't get closer; they just go around. Astronauts in the International Space Station aren't "weightless" because "there's no gravity in space" or anything like that. The Earth's gravitational pull at the ISS is only slightly less than it would be if the ISS was on the ground. What keeps it up there is its speed parallel to the ground. And the astronauts inside are only "weightless" because they are continuously "falling" towards the Earth with the same acceleration as the ISS itself.

    Yes. The Moon also orbits the Sun, of course. It's just that it does it along with the Earth. The actual force that the Sun's gravity exerts on the Moon is about 3 times the force that Earth's gravity exerts on the moon. But Earth is also falling towards the Sun...
     
  14. BdS Registered Senior Member

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    Is there any chance a black holes mass can be torn apart by tidal forces? or some of BH's mass can escape the BH's EH with tidal forces, maybe into an accretion disk or something?
     
    Last edited: Nov 22, 2019
  15. James R Just this guy, you know? Staff Member

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    I don't think so. Mass that is inside the event horizon can't ever escape. Tidal forces are just gravity. They can't pull matter out of a black hole.
     
  16. BdS Registered Senior Member

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    I know it's a low probability according to current theory. With the small radius of the blackhole makes it even less probable, but with the gravity of the rest of the universe against the singular BH gravity maybe it can add up to overpower the local BH's gravitational force?
     
    Last edited: Nov 26, 2019
  17. BdS Registered Senior Member

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    or maybe a BH in a galaxy cluster being forced by the other galaxies of the clusters gravity?

    A tug of gravity war...

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  18. James R Just this guy, you know? Staff Member

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    If it was going to do that, it would have already done it.
     
  19. BdS Registered Senior Member

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    Or is it busy doing it now. The universe must have some mechanism to balance how much mass a single object can contain or everything would be a single object.
    BH's just sucking up mass and holding on to it breaks the logic of what we see in a structured universe like this one...
     
  20. Janus58 Valued Senior Member

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    A black hole has no more gravity than any other object of similar mass, it is just small for its mass meaning objects can get close enough to them to reach the region where gravity is really strong. They do not "suck up mass" any more than anything else. For Any object to keep from being captured by a black hole, all it that has to happen is that its trajectory doesn't pass the event horizon. For an object dropped starting from a great distance away, all this requires is for the object to have a small side component to its velocity. Then it will swing around the black hole in a hyperbolic path and head back into space again. In fact, it is more likely for such an object to hit and be "absorbed" into a 10 solar mass star than a 10 solar mass Black hole. The black hole's event horizon would only make a target with a radius of 30 km, while the star makes one hundreds of thousands of km in radius. An object on a path that would skim past the black hole would hit the surface of the star.

    So the "mechanism" that prevent everything in the universe from being sucked into a single object is simply the fact that the most of the material is too far away and moving to quickly to fall into Black holes. Black holes tend to clear out the stuff nearby and moving slowly, but once they do this, their "fuel supply" dries up for the most part. The quasars that we see in the distant parts (and distant past) of the universe are the Supermassive black holes of early galaxies still clearing out their local neighborhoods. Eventually gather in all the nearby stuff and become more quiescent, like our own galaxies . (Another bit of evidence that black holes aren't just vacuum cleaners that suck up everything is that our galaxy's Central black hole has several stars in stable orbits around it.)

    Along the lines of James R's comment of "If it was going to do it, it would have already done it", While other gravitational sources can have an effect on the black hole in terms of the size of the event horizon, it really boils down to effecting the original formation. In other words, the actually size of the black hole during formation can be influenced by outside gravitational effects. But once it forms, these gravitational forces cannot, in any way extract anything from inside the event horizon. For one thing, there is nothing "hovering" just inside the event horizon waiting for its chance to escape. Everything that enters a black hole is on a one way trip to its center.

    As far as the effects of gravity of other galaxies in a cluster go, If the BH is in the midst of a cluster, then the various gravitational effects of galaxies surrounding it tend to cancel each other out. They do not combine into an outward pulling force. That's not how gravity works.

    So, in the big picture, black holes that take in mass and never let go are not inconsistent with the structured universe we see, and fit in with it quite naturally.

    And while it might not fit in with your understanding of how things work, that just means that there are gaps in your understanding.
     
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  21. BdS Registered Senior Member

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    A big mass with a small surface radius gives external mass access to more of its gravitational potential and greater tidal forces.
    Can the object going back out into space escape the BH's gravitational influence and leave the system completely or would it remain bound in orbit around the BH? The mass of the accretion disk is still considered part of the systems mass to anything at a greater radius than the disk, right? When they say galactic nucleus is the nucleus radius the EH or is the BH, EH and disk considered the galactic nucleus?
    Makes sense, the greater the radius the higher the chance of mass merging with it.
    So the mass that orbits the black hole can be drained out of the system by another BH for example that comes close enough to the disk?
     
  22. James R Just this guy, you know? Staff Member

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    It depends on how close it came to the BH and how fast it's going. Certainly, objects can approach a BH and then escape again, as long as they don't get too close.

    The mass of a black hole is usually taken to refer to the mass inside the event horizon. Since any accretion disc is outside the horizon, it is not technically part of the black hole. But if you want to widen the "system" to include a black hole and its accretion disc, there's no problem with that.

    The centre of our galaxy has many stars orbiting around in different orbits. Right at the centre there's a large black hole with a mass equivalent to millions of Suns. A number of nearby stars orbit that black hole without falling in. (Note: "nearby" is still quite a large distance when you're talking about things on this scale.)

    Theoretically, I guess so, but if two black holes came that close to one another there would be such a mess that they'd probably merge sooner or later.
     
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  23. BdS Registered Senior Member

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    It's not part of the BH, but it is part of the galaxy. If you are outside the galaxy radius you are attracted to the galaxy's BH mass plus all the mass in the disk. Is a galaxy's barycenter always located inside the EH to a mass outside the galaxy? or can the barycenter be located outside the EH?

    Can a accretion disk be influenced by tidal forces? Where the near side of the disk can be accelerated faster than the far side of the disk and cause an extrusion of the disk?
     

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